The determination and analysis of internal (residual) stresses in crystalline solids is an old and well-known art. In essence, the diffraction of X-rays directed at the surface of the material being analyzed comprises the analytical tool by means of which the spacing between adjacent internal atomic planes is determined. This information is collected in the form of diffracted intensity and position data which, in turn, provides the analyst with what is needed to determine internal stresses.
More specifically, the so-called "Bragg Relation" expressed as EQU n.lambda.=2d sin.theta.
where
n=1, 2, . . . , i.e., any integer PA1 .lambda.=wavelength of the diffracted radiation PA1 d=interplanar spacing PA1 .theta.=Bragg angle PA1 1. Scanning Goniometer (Diffractometer) PA1 2. Film Camera PA1 3. Electro-Optical Combinations PA1 4. Position Sensitive Proportional Counters PA1 1. Barrett, C. S., Structure of Metals, 2nd Ed., McGraw-Hill, N.Y., 1952. PA1 2. Cullity, B. D., X-Ray Diffraction, Addison-Wesley, Reading, Mass., 1956
is employed to determine the position and intensity of the diffracted X-ray beam impinging upon the surface of the solid crystalline substance. In those applications where the interplanar spacing (d) is known, either .lambda. or .theta. is known. From the practical standpoint, when using either polycrystalline or powdered materials, .lambda. becomes a constant and 2.theta. is a measured angle. In measuring the latter, however, the diffracted X-ray beam undergoes some diffusion and, therefore, is spread out over a range of from a few tenths to several degrees. This necessitates, of course, ascertaining a mean diffraction angle 2.theta., and such is accomplished by taking intensity measurements at several angular positions.
The prior art includes several different ways of measuring the intensity and position of the diffracted X-ray beam, four such methods being quite widely used. They comprise the following:
The first of these, namely, the scanning goniometer, consists basically of a mechanical turntable upon which the sample can be mounted and oriented at a selected angular position (.theta.) with respect to an incident X-ray beam and at double the chosen angle (2.theta.) with respect to a suitable detector such that the latter is in a position to intercept the diffracted beam. Details concerning the construction and operation of the aforesaid apparatus can be found in any one of the following references:
While the scanning goniometer still constitutes one of the most widely used instruments for determining the position and intensity of a diffracted X-ray beam, it suffers from many shortcomings, not the least of which is its bulk and excessive time per measurement. Moreover, it requires an extremely stable power supply along with complex and highly sophisticated auxiliary instrumentation. Last but by no means least is the fact that it is quite complicated to use and requires service at periodic intervals to keep it operating satisfactorily.
Cameras of one type or another are also used to record both the position and intensity of the diffracted X-ray beam. Certain of these cameras are described in the literature references already listed. While less expensive, simpler and easier to use than the scanning goniometer, most of the cameras developed for this purpose suffer from the inherent disadvantages of being both slow and inaccurate in a field where speed and accuracy are highly desirable.
The remaining two methods used to position and measure the intensity of a diffracted X-ray beam, namely, the electro-optical devices and the conventionally used position-sensitive proportional counters, share the problems of the goniometer in that, while they provide accurate measurements in a reasonably short time, they are both bulky and require mechanical movement to provide internal stress measurement, as well as calling for precise control of the specimen to detector distance. Even so, aside from the device described herein, such methods represent the highest state of the art. A detailed description of the various electro-optical methods presently employed in the location and measurement of the intensity of the diffracted X-ray beam can be found in Green, R. E., "Electro-Optical Systems for Dynamic Display of X-Ray Diffraction Images," Advances in X-Ray Analysis, Vol. 14, Plenum Press, N.Y., 1971. Similarly, an informative description of the position-sensitive proportional counters can be found in James, M. R. and J. B. Cohen, The Application of a Position-Sensitive X-Ray Detector to the Measurement of Residual Stresses," Advances in X-Ray Analysis, Vol. 19, Kendall/Hunt Publishing Co., Dubuque, Iowa, 1976, and in U.S. Pat. No. 4,095,103.
From the foregoing, it should be apparent that the ideal combination of method and instrument for determining the internal (residual) stress in polycrystalline solids via diffracted X-ray beams must, first of all, be fast and accurate. Consistent with the above, it should be easy to use and, preferably, portable. While cost is always a factor of some considerable importance, it may be outweighed by others such as, for example, the elimination of the need for complex and expensive auxiliary equipment and fixtures for applying the instrument (apparatus).
It has now been found in accordance with the teaching of this invention that the shortcomings of the prior art methods and apparatus can, in fact, be overcome while, at the same time, providing the sought-after speed and accuracy contained in a highly portable piece of equipment. Not only is the method combined with a suitable instrument accurate, it is easy for relatively untrained technicians to learn to use. Further, its unique feature of tolerance for variation of the sample to the X-ray sensitive detector surface distance, which is not present in any other similar type of combination of method and instrument, provides for unprecedented speed and convenience of application, without the need of special fixtures or measurement devices.
Accordingly, it is the principal object of the present invention to provide a novel method to calibrate and apply a novel and improved device of the electro-optical, proportional or solid state type, of position-sensitive diffractometers for ascertaining the precise angle of a diffracted X-ray beam while allowing broad tolerances in the sample-to-detector distance.
Another object of this invention is to provide a novel method for use with an electro-optical or proportional types of diffractometers which allows broad tolerances in the distance between the detector and the sample to be measured.
Another object of the invention herein disclosed and claimed is to provide a method for determining the interplanar spacing in a solid crystalline sample that reduces the need for auxiliary apparatus, fixturing, and instrumentation to a near-minimum level.
Yet another objective is to provide an improved method by which the user can quickly ascertain both accurate and reliable data concerning the mean diffraction angle 2.theta. in the Bragg Relation, while allowing broad tolerances in the sample-to-detector distance.
Further objects are to provide methods that an instrument of the class described can be simple to operate, easy to learn to use, compact, free of frequent alignment problems, and one that is readily adaptable for use in most of the common X-ray diffraction applications.